In an effort to understand the molecular mechanisms involved in the transformation process mediated by myelocytomatosis virus (MC29), molecular cloning of the complete proviral genome was undertaken. We have successfully cloned the proviral genome in lambda Charon 21A phage. Studies are now being performed to test the transforming activity of the viral genome using various cell lines derived from mouse, rat, and avian species. In vitro mutagenesis of the MC29 proviral genome is currently being carried out to understand the molecular mechanisms involved in the transformation process mediated by this genome.